Production of the mature gamete in the male involves intrinsic processes of spermatogenesis to produce the sperm and subsequent modifications by extrinsic processes in the epididymis to make the sperm able to fertilize. One area of study is spermatogenic cell gene expression and the identification of lineage-specific and stage-specific germ cell proteins. The hypothesis being tested is that these gene products are responsible for the unique structural and functional characteristics of germ cells. The flagellum is a unique structure of sperm and the hypothesis predicts that it contains germ cell-specific proteins. Although germ cells have been reported to lack intermediate filament (IF) proteins, we have identified cytoskeletal proteins in the flagellum that appear to be germ cell-specific IF proteins. Monoclonal antibodies have been used to characterize 67kD and 78kD proteins in the fibrous sheath of the mouse sperm flagellum that are synthesized during spermatogenesis and share antigenic determinants with IF proteins. However, they also contain unique epitopes and have molecular weights and other characteristics unlike known IF proteins. The fibrous sheath is important in determining effectiveness of the flagellar beat, and men with abnormalities in this structure are infertile. The genes for these proteins will be cloned to determine their structure and homology with known IF proteins. The other area of study is germ cell- somatic cell interaction and the identification of somatic cell products required for sperm maturation. A factor produced by Sertoli cells stabilizes binding of epididymal secretory products to the sperm surface. Dilution of this factor in the female productive tract allows the surface components to be shed during capacitation. The factor is present in Sertoli cell-conditioned medium and an assay has been developed to monitor its purification. Isolation of the factor and preparation of monoclonal antibodies will allow studies of the synthesis of the factor and the nature of its interaction with sperm surface molecules.